A Thesis Submitted in Fulfilment of the Requirements for the Degree of Doctor of
Philosophy in Life Sciences of the Nelson Mandela African Institution of Science and
Technology
Bacterial wilt disease (BWD), caused by Ralstonia solanacearum (Smith) is one of the most
destructive tomato diseases globally. A research was carried out from 2017 to 2019 to assess
the incidence and severity of tomato BWD, characterize the pathogen and evaluate effects of
selected pesticidal plants against BWD. Results indicated that 55.17% of the surveyed fields
in different districts in Tanzania were significantly (p ≤ 0.05) infected by BWD with overall
disease incidences and severities ranging from 5.8 to 44.6% and 10.70 to 59.30%
respectively. Majority (70%) of farmers used synthetic chemicals, 13% used botanical, 10%
did crop rotation and 7% did not use any BWD management measure. A total of 29 Ralstonia
isolates were isolated from infected tomato plants out of which 19 (52%) were pathogenic on
tomato variety Tanya. Carbohydrate oxidation test showed that most (90%) predominating
isolates in the main agro-ecological zones belong to biovar 3 while the rest (10%) belong to
biovar 2 and prevail in the southern zone of Tanzania. This is the first report of prevalence of
biovar 2 of Ralstonia in Tanzania and it alerts global plant health regulators to prevent new
introduction and/or spread of Ralstonia. Phylogenetic analysis based sequence data
generated from the 16S-23S internal transcribed spacer (ITS) of isolates revealed that most
(80%) were phylotype I and III and the rest (20%) were phylotype II of R.
pseudosolanacearum and R. solanacearum geno-species respectively. Such results implied
that BWD in the country is caused by diverse Ralstonia populations. In screening the effect of
20 plant extracts, the findings revealed that R. solanacearum can be managed using extracts
from one of the tested plants (Patent Application ID Number G190916-2061) which was as
effective as the positive control (ampiclox) in inhibiting growth of Ralstonia in in-vitro
conditions. It produced inhibition zone of 19.25 mm almost similar to 20.58 mm by the
ampiclox compared with 0.00 mm of the negative control. Similarly, the application of the
extracts of the most effective plant managed BWD severity on tomato seedlings grown in
screen-house 100% compared with the untreated seedlings which wilted. Therefore the
extract from the most effective plant will be recommended after patenting for developing
effective and sustainable management strategies of BWD in Tanzania based on the pathogen
characteristics as generated during this study.